The production of light olefins (ethylene, propylene and butenes) from various hydrocarbon feedstocks utilizes the technique of pyrolysis, or steam cracking. Pyrolysis involves heating the feedstock sufficiently to cause thermal decomposition of the larger molecules. The pyrolysis process, however, produces molecules which tend to combine to form high molecular weight materials known as tars. Tars are high-boiling point, viscous, reactive materials that can foul equipment under certain conditions. Although not wishing to be bound by any particular theory, it is believed that the steam cracked liquid product, as first produced in the steam cracker furnace, contain free radical molecules, vinyl-aromatic molecules, and other reactive species, and is highly reactive at moderately high temperatures commonly found in the downstream processing of steam cracked liquid product. The unsaturated functional groups of such aromatic molecules include those selected from the group consisting of olefinic groups and acetylenic groups. More specifically, such unsaturated functional groups are selected from the groups consisting of indenes, acenapthalenes and other cyclopenteno-aromatics; vinylbenzenes, and other vinyl aromatics having one aromatic ring; divinylbenzenes, vinylnaphthalenes, divinylnaphthalenes, vinylanthracenes, vinylphenanthrenes, and other vinyl- and divinylaromatics having 2 or more aromatic rings. This reactivity of such aromatic molecules tends to lead to reactions which significantly downgrade the properties of the liquid product.
The formation of tars, after the pyrolysis effluent leaves the steam cracking furnace can be minimized by rapidly reducing the temperature of the effluent exiting the pyrolysis unit to a level at which the tar-forming reactions are greatly slowed.
One technique used to cool pyrolysis unit effluent and remove the resulting heavy oils and tars employs heat exchangers followed by a water quench tower in which the condensibles are removed. This technique has proven effective when cracking light gases, primarily ethane, propane and butane, because crackers that process light feeds, collectively referred to as gas crackers, produce relatively small quantities of tar. As a result, heat exchangers can efficiently recover most of the valuable heat without fouling and the relatively small amount of tar can be separated from the water quench albeit with some difficulty.
This technique is, however, not satisfactory for use with steam crackers that crack naphthas and heavier feedstocks, collectively referred to as liquid crackers, since liquid crackers generate much larger quantities of tar than gas crackers. Heat exchangers can be used to remove some of the heat from liquid cracking, but only down to the temperature at which tar begins to condense. Below this temperature, conventional heat exchangers cannot be used because they would foul rapidly from accumulation and thermal degradation of tar on the heat exchanger surfaces. In addition, when the pyrolysis effluent from these feedstocks is quenched, some of the heavy oils and tars produced have approximately the same density as water and can form stable oil/water emulsions. Moreover, the larger quantity of heavy oils and tars produced by liquid cracking would render water quench operations ineffective, making it difficult to raise steam from the condensed water and to dispose of excess quench water and the heavy oil and tar in an environmentally acceptable manner.
Accordingly, in most commercial liquid crackers, cooling of the effluent from the cracking furnace is normally achieved using a system of transfer line heat exchangers, a primary fractionator, and a water quench tower or indirect condenser. For a typical heavier than naphtha feedstock, the transfer line heat exchangers cool the process stream to about 1100° F. (594° C.), efficiently generating super-high pressure steam which can be used elsewhere in the process. The primary fractionator is normally used to condense and separate the tar from the lighter liquid fraction, known as pyrolysis gasoline, and to recover the heat between about 200° to 600° F. (93° to 316° C.). The water quench tower or indirect condenser further cools the gas stream exiting the primary fractionator to about 100° F. (38° C.) to condense the bulk of the dilution steam present and to separate pyrolysis gasoline from the gaseous olefinic product, which is then sent to a compressor. Sometimes an intermediate boiling range stream known as steam cracked gas oil boiling, say, within the range of about 400° to about 550° F. (204° to 288° C.), is also produced as a sidestream.
Moreover, despite the fractionation that takes place between the tar and gasoline streams in a primary fractionator, both streams often need to be processed further. Sometimes the tar needs to be stripped to remove light components, whereas the gasoline may need to be refractionated to meet its end point specification. An additional concern relates to providing steam cracked tar having characteristics which make it suitable for high value use.
Steam cracker tar is the heaviest material made in the steam cracking process, comprising essentially all the product that boils above about 500° F. (260° C.). Such tar contains a high concentration of aromatic compounds produced by chemical reactions which lead to molecular weight growth of steam cracked liquids, e.g., condensation and/or polymerization reactions in the cracking process. These reactions can occur to a large extent in the primary fractionator or quench tower at the temperatures that normally prevail in steam cracker primary fractionator towers. These molecular weight growth reactions leading to asphaltene formation are rather fast and are not as easily reversed as they are prevented.
The yield of tar depends primarily on the cracker feed type, e.g., about 1 wt % from naphtha and 30% or more from very heavy gas oil. The value of tar is generally based on its use as a fuel or fuel blend stock. Sometimes it can be used as a feedstock for making carbon black. Tar can also be fed to a partial oxidation process where it is converted to synthetic fuel gas.
Molecules in tar containing more than about seven aromatic rings are insoluble in heptane and are known as asphaltenes. Asphaltenes are high molecular weight, complex aromatic ring structures and may exist as colloidal dispersions. With their aromatic ring structure, asphaltenes are not soluble in straight chain alkanes (hexane, heptane). They are soluble in aromatic solvents like xylene and toluene. Asphaltene content can be measured by various techniques known to those of skill in the art, e.g., ASTM D3279.
The heavier molecules in tar that are not soluble in toluene are known as toluene insolubles, or TI. Toluene Insolubles (coagulated/uncoagulated) are the solids remaining after oxidation resins, or pentane insolubles, have been diluted with toluene. Insoluble resins are the difference in weight between the pentane insolubles and the toluene insolubles. Toluene insolubles can be measured by methods well known to those skilled in the art, e.g., ASTM D-893, ASTM D4312-05(a)2005, Standard Test Method for Toluene-Insoluble (TI) Content of Tar and Pitch (Short Method), or ASTM D4072-98(2003)el, Standard Test Method for Toluene-Insoluble (TI) Content of Tar and Pitch.
Asphaltenes and TI affect the quality and resulting value of the tar in several ways. They make steam cracker tar incompatible with many other fuel oils. For example, asphaltenes tend to precipitate when tar is mixed with paraffinic stocks, such as residua from paraffinic crude oil. This limits the potential marketability of tar into the fuel oil market. Moreover, asphaltenes and TI are not desirable components when tar is used in the manufacture of carbon black. Carbon black producers generally prefer feeds with lower asphaltene and TI concentrations, and they set upper limits on acceptable concentrations of these components.
Because asphaltenes and TI make tar more viscous, it often becomes necessary to mix a lighter aromatic material such as steam cracked gas oil with the tar, in order to meet product viscosity specifications. For crackers that feed naphtha or highly paraffinic gas oil, the amount of light blend stock required can exceed the quantity of co-produced steam cracked gas oil, which renders the steam cracking process “out of quench balance” inasmuch as the quantity of light blend stock produced in the cracker is insufficient to thin produced steam cracker tar to its desired viscosity. In such cases, an external source of light, highly aromatic material must be added, and this can be difficult to obtain and costly. Alternately, cracking severity must be reduced which imposes yield and conversion restrictions on the steam cracking process.
In view of the foregoing, it would be useful to provide a method for treating pyrolysis unit effluent, particularly the effluent from the steam cracking of hydrocarbonaceous feeds include naphtha and heavier feeds which yield greater amounts of steam cracker tar than lighter feeds. Accordingly, it would be useful to provide a steam cracking process which produces steam cracker tar having a reduced asphaltenes and/or toluene insolubles content, particularly where the process can be carried out in the presence or absence of a primary fractionator tower and its ancillary equipment, e.g., in processes utilizing a tar knock-out drum.
U.S. Pat. Nos. 4,279,733 and 4,279,734 propose cracking methods using a quencher, indirect heat exchanger and fractionator to cool effluent, resulting from steam cracking.
U.S. Pat. Nos. 4,150,716 and 4,233,137 propose a heat recovery apparatus comprising a pre-cooling zone where the effluent resulting from steam cracking is brought into contact with a sprayed quenching oil, a heat recovery zone, and a separating zone.
Lohr et al., “Steam-cracker Economy Keyed to Quenching,” Oil Gas J., Vol. 76 (No. 20) pp. 63-68 (1978), proposes a two-stage quenching involving indirect quenching with a transfer line heat exchanger to produce high-pressure steam along with direct quenching with a quench oil to produce medium-pressure steam.
U.S. Pat. Nos. 5,092,981 and 5,324,486 propose a two-stage quench process for effluent resulting from steam cracking furnace comprising a primary transfer line exchanger which functions to rapidly cool furnace effluent and to generate high temperature steam and a secondary transfer line exchanger which functions to cool the furnace effluent to as low a temperature as possible consistent with efficient primary fractionator or quench tower performance and to generate medium to low pressure steam.
U.S. Pat. No. 5,107,921 proposes transfer line exchangers having multiple tube passes of different tube diameters. U.S. Pat. No. 4,457,364 proposes a close-coupled transfer line heat exchanger unit.
U.S. Pat. No. 3,923,921 proposes a naphtha steam cracking process comprising passing effluent through a transfer line exchanger to cool the effluent and thereafter through a quench tower.
WO 93/12200 proposes a method for quenching the gaseous effluent from a hydrocarbon pyrolysis unit by passing the effluent through transfer line exchangers and then quenching the effluent with liquid water so that the effluent is cooled to a temperature in the range of 220° to 266° F. (105° to 130° C.), such that heavy oils and tars condense, as the effluent enters a primary separation vessel. The condensed oils and tars are separated from the gaseous effluent in the primary separation vessel and the remaining gaseous effluent is passed to a quench tower where the temperature of the effluent is reduced to a level at which the effluent is chemically stable.
EP 205 proposes a method for cooling a fluid such as a cracked reaction product by using transfer line exchangers having two or more separate heat exchanging sections.
U.S. Pat. No. 5,294,347 proposes that in ethylene manufacturing plants, a water quench column cools gas leaving a primary fractionator and that in many plants, a primary fractionator is not used and the feed to the water quench column is directly from a transfer line exchanger.
JP 2001-40366 proposes cooling mixed gas in a high temperature range with a horizontal heat exchanger and then with a vertical heat exchanger having its heat exchange planes installed in the vertical direction. A heavy component condensed in the vertical exchanger is thereafter separated by distillation at downstream refining steps.
WO 00/56841; GB 1,390,382; GB 1,309,309; and U.S. Pat. Nos. 4,444,697; 4,446,003; 4,121,908; 4,150,716; 4,233,137; 3,923,921; 3,907,661; and 3,959,420; propose various apparatus for quenching a hot cracked gaseous stream wherein the hot gaseous stream is passed through a quench pipe or quench tube wherein a liquid coolant (quench oil) is injected.
U.S. Pat. No. 5,215,649 teaches a method for upgrading steam cracker tars by injecting hydrogen donor diluent into a hot cracked product stream at a point downstream of a point where high temperature cracking is stopped by cooling.